1. Field of Invention
The present invention relates to a developer apparatus and to an image forming apparatus wherein developer material is transported by a traveling-wave electric field and a latent electrostatic image is developed by means of this developer material.
2. Conventional Art
In the field of copiers, printers, and other such image forming apparatuses where electrophotography is employed, developer apparatuses utilizing noncontact methods in which developer material is transported to the vicinity of an image carrier and developer material is cast onto a latent electrostatic image on the image carrier to develop this latent electrostatic image have drawn attention. Such noncontact methods include the powder cloud method, the jumping method, and methods employing an electric field curtain (traveling-wave electric field).
Methods employing traveling-wave electric fields are described, for example, at Japanese Patent Application Publication Kokoku No. H5-31146 (1993), Japanese Patent Application Publication Kokoku No. H5-31147 (1993), and elsewhere. In such descriptions, a multiplicity of electrodes are embedded in a developer material transport path, polyphase AC voltage(s) is or are applied to these electrodes to form a traveling-wave electric field, and developer material in the transport path is transported to an image carrier by means of this traveling-wave electric field. Developer material transported to the vicinity of the image carrier and cast onto a latent electrostatic image on the image carrier adheres to the latent electrostatic image. As a result, the latent electrostatic image on the image carrier is developed.
Furthermore, at Japanese Patent Application Publication Kokai No. H3-21967 (1991), not only is developer material in a transport path transported by a traveling-wave electric field, but art is also disclosed in which a precharge roller made of urethane foam and a blade that contacts the precharge roller are provided, friction between the precharge roller and the transport path causing precharging of developer material while developer material layer thickness is at the same time restricted, as a result of which uniform and appropriate charging, as well as stable transport, of developer material are achieved, while scattering of developer material and fogging of the image are prevented.
However, as a result of intensive research on the part of the inventors of the present invention, it has been found that the foregoing conventional developer apparatuses have problems such as the following.
The traveling-wave electric field for transport of developer material is formed due to differences in electric potential between the respective electrodes of the transport path and the developer material supply member which supplies the developer material to the transport path. For this reason, it is necessary to not only apply AC voltage(s) Vac to the electrodes of the transport path but to also apply prescribed DC bias voltage(s) Vd to the developer material supply member, as shown at FIG. 15(a). Furthermore, where the developer material supply member is additionally outfitted with restricting members for restricting developer material layer thickness, supplemental supply members for smooth supply of developer material, and so forth, it will be necessary to apply DC voltage(s) to the restricting members, supplemental supply members, and so forth so as to respectively bias these relative to the DC bias voltage Vd at the developer material supply member.
Now, the developer material becomes charged through ionic irradiation by a corona discharge device, immersion in an electric field, triboelectric action, or the like. However, the amount of charge acquired will vary depending upon ambient conditions and will vary as a function of time. Similarly, developer material layer thickness (the amount of developer material adhering to media) will also vary. Such variations in developer material contribute to variation in the amount of developer material supplied from the developer material supply member to the transport path, and therefore to variation in the amount of developer material supplied from the transport path to the image carrier, causing development nonuniformity and interfering with stable image formation.
One proposal for increasing stability of the amount of developer material which is supplied is a method wherein the traveling-wave electric field for transport of developer material is varied. For example, if there is a decrease in the amount of developer material being supplied, the difference in electric potential between AC voltage Vac and the DC bias voltage Vd at the developer material supply member might be increased by raising DC bias voltage Vd as shown in FIG. 15(b) and/or lowering AC voltage Vac as shown in FIG. 15(c), thereby increasing the intensity of the traveling-wave electric field and causing the amount of developer material being supplied to increase.
Where AC voltage Vac is varied as shown in FIG. 15(c), however, the fact that it will be necessary to uniformly vary at least three or four phases of high-voltage AC voltage makes for complicated voltage supply circuitry for supply of the high-voltage AC voltage(s), which leads to increased cost. And if a relative shift were to develop among the respective high-voltage AC voltages, transport of developer material would become destabilized and the amount of developer material being supplied would likewise become destabilized. Accordingly, in addition to the fact that voltage supply circuitry is made complicated by additional equipment in the form of a mechanism for varying the respective high-voltage AC voltages, as stable operation of the voltage supply circuitry must be maintained and as it will be necessary to simultaneously achieve both stable operation as well as a mechanism for varying respective high-voltage AC voltages, increases in cost will be unavoidable.
Furthermore, where the DC bias voltage Vd at the developer material supply member is varied as shown in FIG. 15(b), as it will also be necessary, in conjunction with variation of the DC bias voltage Vd, to vary the respective DC bias voltages at the aforementioned restricting members for restricting developer material layer thickness, supplemental supply members for smooth supply of developer material, and so forth, here again this will complicate the voltage supply circuitry for supply of respective DC bias voltages, increasing cost. Furthermore, because variation of the respective DC bias voltages at such members will result in variation in the electric field distribution in the vicinity of the developer material transport path, it is entirely possible that this will produce unexpected behavior in the development process or affect transport of developer material.
It is therefore an object of the present invention to provide a developer apparatus and an image forming apparatus conceived in light of the foregoing problems in the conventional art and permitting adjustment in the amount of developer material supplied through a simple constitution to achieve improved stability in image formation while holding increases in cost to a minimum.
In order to solve the foregoing problems, the present invention, in the context of a developer apparatus equipped with one or more transport path or paths wherein a plurality of electrodes are arranged in a row or rows so as to be mutually separated by a prescribed spacing or spacings and with one or more developer material supply means arranged at the front side of at least one of the transport path or paths, developer material being supplied from at least one of the developer material supply means to the front of at least one of the transport path or paths, a polyphase alternating current voltage or voltages being applied to respective electrodes of at least one of the transport path or paths, a traveling-wave electric field or fields being formed, at least one of the traveling-wave electric field or fields causing at least a portion of the developer material to be transported along the front of at least one of the transport path or paths to an image carrier or carriers, and supply of this developer material to the image carrier or carriers causing a latent electrostatic image or images on at least one of the image carrier or carriers to be developed, is such that a rear electrode or electrodes is or are arranged at a location or locations at the back side of at least one of the transport path or paths opposite at least one of the developer material supply means, a developer-material-supplying electric field or fields being formed between at least one of the rear electrode or electrodes and at least one of the developer material supply means.
A developer apparatus having such constitution according to the present invention permits formation of developer-material-supplying electric field(s) between developer material supply mean(s) and rear electrode(s) at location(s) at the back side(s) of transport path(s). Accordingly, developer-material-supplying electric field(s) will be formed near developer material supply path(s) between developer material supply mean(s) and transport path(s) and will exert an effect upon the amount(s) of developer material supplied. Furthermore, intensity or intensities of developer-material-supplying electric field(s) may be adjusted by altering voltage(s) applied to rear electrode(s). Amount(s) of developer material supplied from developer material supply mean(s) to transport path(s) may therefore be controlled by altering voltage(s) applied to rear electrode(s) and adjusting intensity or intensities of developer-material-supplying electric field(s). This eliminates the need to vary DC bias voltage(s) at developer material supply mean(s) and/or polyphase AC voltage(s) applied to respective electrodes in transport path(s), therefore making it possible to avoid complicated voltage supply circuitry for supply of polyphase AC voltage(s) and DC bias voltage(s) and concomitant increases in cost, and moreover permitting achievement of improved stability in image formation without destabilizing transport of developer material or producing unexpected behavior in the development process or effect on transport of developer material.
Furthermore, in the present invention, a width of at least one of the rear electrode or electrodes in at least one developer material transport direction is greater than a pitch or pitches between respective electrodes in at least one of the transport path or paths.
If width(s) of rear electrode(s) were to be made smaller than pitch(es) between respective electrodes in transport path(s), developer-material-supplying electric field(s) produced by rear electrode(s) would be more or less shielded by respective electrodes in transport path(s), making it impossible to use developer-material-supplying electric field(s) to control amount(s) of developer material supplied. Width(s) of rear electrode(s) are therefore made greater than pitch(es) between respective electrodes in transport path(s).
Moreover, in the present invention, at least one of the rear electrode or electrodes is disposed with a bias in at least one developer material transport direction relative to at least one of the developer material supply means.
Arranging rear electrode(s) in such fashion causes developer-material-supplying electric field(s) produced by rear electrode(s) to be biased in developer material transport direction(s) relative to developer material supply mean(s). In such a case, it is possible for developer material to be smoothly directed from developer material supply mean(s) to transport path(s), improving developer material transport stability. If rear electrode(s) were disposed with bias(es) in opposite direction(s) relative to developer material supply mean(s), developer-material-supplying electric field(s) produced by rear electrode(s) would be biased in opposite direction(s) relative to developer material supply mean(s), increasing the tendency for developer material to become concentrated at location(s) to the front of region(s) between developer material supply mean(s) and transport path(s), causing developer material itself to block developer material transport path(s) at such locations and causing developer material to no longer be able to smoothly pass between developer material supply mean(s) and transport path(s), and destabilizing developer material transport.
Furthermore, in the present invention, a length of at least one of the rear electrode or electrodes in a direction perpendicular to at least one developer material transport direction is less than a length or lengths of respective electrodes in at least one of the transport path or paths in said perpendicular direction.
For each of the several phases of the polyphase AC voltage(s), respective electrodes of transport path(s) are connected in common and the AC voltage(s) is or are applied to the respective electrodes connected in common. The region of the respective electrodes at which they are connected in common is the ends of the respective electrodes. For this reason, the pattern formed by the ends of respective electrodes is made complex, the traveling-wave electric field(s) produced by the respective electrodes being disrupted in the region of this complex pattern. Accordingly, transport of developer material is destabilized at the ends of respective electrodes, it being preferred that transport of developer material not take place thereat. Length(s) of rear electrode(s) is or are therefore made smaller than length(s) of respective electrodes, inhibiting transport of developer material in the vicinity of the ends of respective electrodes, there being no supply of developer material to the vicinity of the ends of respective electrodes.
Moreover, in the present invention, at least one of the developer-material-supplying electric field or fields is an alternating electric field.
Developer material tends to accumulate in layers and adhere to developer material supply mean(s). For this reason, alternating electric field(s) is or are chosen for use as developer-material-supplying electric field(s), developer material layers being broken up by the periodic variation between high and low developer-material-supplying electric field intensities. This permits supply of developer material to be made uniform and stable. Also, while traveling-wave electric field(s) comprises or comprise a plurality of alternating electric field(s), the frequency or frequencies, electric field intensity or intensities, phase difference(s), and so forth thereof are optimized for transport of developer material. Accordingly, it is desirable that, completely separate from traveling-wave electric field(s), alternating electric field(s) representing developer-material-supplying electric field(s) be such that the frequency or frequencies and/or electric field intensity or intensities thereof is or are optimized for uniform and stable supply of developer material.
Furthermore, in the present invention, an alternating current voltage or voltages corresponding to the alternating electric field is or are applied to at least one of the rear electrode or electrodes.
If AC voltage(s) corresponding to alternating electric field(s) were applied to developer material supply mean(s), such alternating electric field(s) would also act at transport path(s) in the vicinity or vicinities of developer material supply mean(s). Or such alternating electric field(s) might also act at restricting members for restricting developer material layer thickness, supplemental supply members for smooth supply of developer material, and so forth. This might then cause problems with layer formation of developer material being transported along the front(s) of transport path(s). It is moreover possible that action of such alternating electric field(s) could extend as far as the vicinity or vicinities of development region(s) where latent electrostatic image(s) on image carrier(s) is or are being developed, and if electric field(s) in the vicinity or vicinities of such development region(s) is or are disrupted this would negatively affect the development process. AC voltage(s) corresponding to alternating electric field(s) is or are therefore applied to rear electrode(s), causing region(s) at which such alternating electric field(s) is or are produced to be concentrated between rear electrode(s) and developer material supply mean(s), and inhibiting action of such alternating electric field(s) at regions peripheral thereto.
Moreover, in the present invention, the condition (L/xcex)xc3x97(1/(Nxc3x97f2)) greater than 1/f1 is satisfied, where f1 is a frequency of the alternating electric field, N is a number of phases of at least one of the polyphase alternating current voltage or voltages which forms or form at least one of the traveling-wave electric field or fields, f2 is a frequency of at least one of the traveling-wave electric field or fields, L is a width of at least one of the rear electrode or electrodes in at least one developer material transport direction, and xcex is at least one of the pitch or pitches between respective electrodes in at least one of the transport path or paths.
Taking the case of two adjacent electrodes in a transport path, the time during which developer material is moving between said respective electrodes corresponds to the time during which an electric potential difference exists between said respective electrodes. For this reason, taking the example where polyphase AC voltage(s) is or are four-phase, choosing four rectangular waves mutually differing in phase by 90xc2x0 and having duty cycles of 50% or more for use as four-phase AC voltage(s) maximizes the time during which an electric potential difference exists between two adjacent electrodes and increases the time during which movement of developer material occurs. Here, the time during which developer material is moving across the space between two adjacent electrodes will be 1/(Nxc3x97f2), where N is the number of phases of polyphase AC voltage and f2 is traveling-wave electric field frequency (Hz). Furthermore, there will be L/xcex spaces between respective electrodes within rear electrode region(s), where L is rear electrode width (m) and k is pitch (m) between respective electrodes in a transport path. Accordingly, xcex94t=(L/xcex)xc3x97(1/(Nxc3x97f2)), where xcex94t is the time during which developer material is moving in rear electrode region(s). Moreover, in order that alternating electric field(s) representing developer-material-supplying electric field(s) act on developer material for at least one cycle in rear electrode region(s), and to thus promote uniformity and stability in supply of developer material, it will be necessary to make xcex94t greater than alternating electric field period (1/f1), where f1 is alternating electric field frequency (Hz). Accordingly, if the condition (L/xcex)xc3x97(1/(Nxc3x97f2)) greater than 1/f1 is satisfied, supply of developer material will be made uniform and stable, and image formation will in turn be made stable.
In addition, where polyphase AC voltage(s) is or are three-phase, three rectangular waves mutually differing in phase by 90xc2x0 and having duty cycles of 50% or more may be chosen for use as three-phase AC voltage(s).
Furthermore, in the present invention, supply of developer material from at least one of the developer material supply means to the front of at least one of the transport path or paths is stopped by switching at least one of the developer-material-supplying electric field or fields to a non-developer-material-supplying electric field.
Stopping supply of developer material from developer material supply mean(s) to transport path(s) in mid-supply thereof causes binding of developer material layer(s) at the front(s) of transport path(s), and this negatively affects supply of developer material the next time that supply thereof is attempted. This might for example deleteriously affect attempts to increase uniformity and stability of supply, or vibrations from the exterior might serve to dislodge and scatter developer material layer(s). Supply of developer material to transport path(s) is therefore stopped through use of non-developer-material-supplying electric field(s). If developer material in transport path(s) is transported in such fashion without leaving any of it unrecovered, binding of developer material layer(s) at the front(s) of transport path(s) can be avoided. And not only that, but because switching from developer-material-supplying electric field(s) to non-developer-material-supplying electric field(s) is carried out by merely switching voltages applied at rear electrode(s), such effect may be achieved simply and inexpensively.
Moreover, in the present invention, the condition d1 greater than d2 is satisfied, where d1 is a distance separating at least one of the rear electrode or electrodes and respective electrodes of at least one of the transport path or paths, and d2 is a distance separating respective electrodes of at least one of the transport path or paths and the front of at least one of the transport path or paths.
If distance(s) d1 separating rear electrode(s) and respective electrodes of transport path(s) is or are too small, there will be an increase in the degree to which traveling-wave electric field(s) produced by respective electrodes is or are directed toward rear electrode(s), reducing traveling-wave electric field intensity or intensities and reducing developer material transport capability. Distance(s) d2 separating respective electrode(s) of transport path(s) and the front(s) of transport path(s) is or are therefore made smaller than distance(s) d1 separating rear electrode(s) and respective electrodes of transport path(s), this permitting traveling-wave electric field intensity or intensities to be maintained.
Furthermore, in the present invention, the condition Bs greater than d1 is satisfied, where Bs is a distance separating respective electrodes of at least one of the transport path or paths, and d1 is a distance separating at least one of the rear electrode or electrodes and respective electrodes of at least one of the transport path or paths.
If distance(s) Bs separating respective electrodes of transport path(s) is or are too small relative to distance(s) d1 separating rear electrode(s) and respective electrodes of transport path(s), or if distance(s) d1 is or are too large relative to distance(s) Bs, developer-material-supplying electric field(s) produced by rear electrode(s) will be more or less shielded by respective electrodes in transport path(s), making it impossible to use developer-material-supplying electric field(s) to control amount(s) of developer material supplied. Distance(s) Bs separating respective electrodes of transport path(s) is or are therefore made larger than distance(s) d1 separating rear electrode(s) and respective electrodes of transport path(s).
Moreover, an image forming apparatus in accordance with the present invention is equipped with at least one developer apparatus as described above.
Such an image forming apparatus in accordance with the present invention also permits attainment of operation and benefits similar to those described with respect to the foregoing developer apparatus(es).